Thickness shear vibration type, crystal electromechanical filter

ABSTRACT

1,023,671. Piezo-electric crystals. TOYO TSUSHINKI KABUSHIKI KAISHA. Dec. 6, 1962 [Dec. 27,1961], No. 46134/62. Heading H1E A lattice filter is realized by means of a crystal plate having divided electrodes and vibrating in two fundamental thickness-shear modes. The plate is an AT or BT quartz cut, the major faces of such a plate being in the XZ plane. It is stated in the Specification that one fundamental thickness-shear mode exhibits a displacement distribution with one sine wave in the X direction, one cosine wave in the Y a  direction and zero displacement in the Z a  direction. Such a mode may be designated (1.1.0). Two further so-called inharmonic modes can be expressed as (2.1.0) and (1.1.1), these having similar sine and cosine distributions. In the filter of the invention, the fundamental mode and one of the inharmonic modes are excited in the plate. The plates of Figs, 2A, 2B utilize the (1.1.0) and (2.1.0) modes and have electrodes divided in the Z a  direction so that transmission is in the X direction. By minimizing the Z a  dimension of the plate as in Fig. 2B, the (1.1.1) mode can be suppressed; in the plate of Fig. 2A this mode is present but does not affect the electrical input or output. In the circular plates of Figs. 2D, 2E suppression of unwanted modes is achieved by means of slits 20. By dividing the electrodes in the X direction (so that transmission is in the Z a direction) a plate utilizing the (1.1.0) and (1.1.1) modes may be realized. In both types of plates the two modes of vibration have an antiphase relationship and this property is utilized in the design of a bandpass filter. The Specification describes in detail the equivalent lattice network for a plate vibrating in each of the modes discussed and it is shown how the equivalent reactances are related to electrode distribution and crystal size. The crystal is dimensioned so that the resonance frequencies of the fundamental and the inharmonic modes are adjacent, the difference being one-half of the band-width. A three-terminal filter may be realized by connecting one pair of divided electrodes to a common terminal as shown in Fig. 4A. Filters may be connected in cascade and capacitances connected across input and output terminals to provide attenuation poles.

Aug. 6, 1968 YUZO NAKAZAWA 3,396,327

THICKNESS SHEAR VIBRATION TYPE, CRYSTAL ELECTROMECHANICAL FILTER FiledDec. 5, 1962 6 Sheets-Sheet 2 6, 1968 YUZO NAKAZAWA 3,396,327

THICKNESS SHEAR VIBRATION TYPE, CRYSTAL ELECTROMECHANICAL FILTER FiledDec. 5, 1962 3 Sheets-Sheet 3 (A) xlfC 11 A x 50 n A i 1, z x X 3'0\x/l/ 3,396,327 THICKNESS SHEAR VIBRATION TYPE, CRYSTALELECTROMECHANICAL FILTER Yuzo Nakazawa, Kohoku-ku, Yokohama-sin,Kanagawaken, Japan, assignor to Toyotsushinki Kabushiki Kalsha,Tsukakoshi, Kawasaki-shi, Japan, a joint-stock company of Japan FiledDec. 3, 1962, Ser. No. 241,747 Claims priority, application Japan, Dec.27, 1961, 36/ 47,189 6 Claims. (Cl. 333-72) ABSTRACT OF THE DISCLOSURE Ahigh frequency electromechanical band pass filter comprises a thinpiezoelectric crystal having an X-axis (electrical axis), Y-axis(mechanical axis) and Z-axis (optical axis) and having thecharacteristic of vibrating in the thickness shear mode of vibrationwith substantially all vibrational displacements in the X-axisdirection. The crystal has parallel major faces and a plurality ofelectrodes on the major faces of the crystal. The electrodes comprise aninput electrode and output electrode on. the same face divided from oneanother along a center line perpendicular to the X-axis with paralleledges of the electrodes spaced from one another. The crystal may becircular with the electrodes semicircular and of lesser radius than thecrystal. Notches provided in the perimeter of the crystal in line withthe division between the electrodes preferably extend into the perimeterof the electrodes,

This invention relates to electromechanical filters, and moreparticularly it relates to a new thickness shear vibration type, crystalelectromechanical filter having highly advantageous characteristics.

In general, an electromechanical filter comprises an electromechanicaltransducer which converts electrical energy into mechanical energ amechanical filter which filters the converted mechanical energy, and anelectromechanical transducer which converts the filtered transmissionenergy into electrical energy.

It is an object of the present invention to provide a new crystalelectromechanical filter of the so-called high-frequency type operatingin a frequency range of from sev eral to several tens of megacycles persecond, wherein is used, for the above-mentioned elements, apiezoelectric quartz crystal unit, its characteristic of functioningalso as an electromechanical transducer being utilized, and wherein athickness shear vibration =rnode suitable for high frequencies is used.

The so-called AT and BT cut crystal vibrators with zero temperaturecoefiicients have, in general, numerous possible resonance frequencies.Of these, two modes of vibrations are principally used. The frequenciesof vibration produced by the thickness shear mode are not affectedsubstantially by the shape and dimensions of the plate, although thecrystal plate may be given a variety of shapes, rectangle, square,circle, rhombus, etc., and its resonance frequency gradually increaseswith decreasing dimensions. On the other hand, in the case of thesocalled higher flexural mode, its resonance frequency is seriouslyaffected by the shape and dimensions of the plate, increasing abruptlywith decreasing dimensions.

The crystal electromechanical filter of the character to which thisinvention relates, is based on two particular vibrations of thethickness shear vibratiton modes.

The principle and nature of the inventiton will be more clearly apparentby reference to the following detailed description when taken inconjunction with the accompanyin drawings in which:

3,396,327 Patented Aug. 6, 1968 FIG. 1 consists of views for describingthickness shear vibration modes of crystal vibrators, wherein FIG. 1(A)presents views indicating the electrical polarization distribution ofthe main vibration, and FIG. 1(B) and FIG. 1(C) present views indicatingthe electrical polarization distributions of vibrations utilized in thefilter and indicating the principle of the invention; and wherein, ineach of FIG. 1(A), 1(3), and 1(C), (I) is a perspective view, and (II)is a cross sectional view;

FIG. 2(A) is a perspective view showing one embodiment of the presentinvention;

FIG. 2(B) I and FIG. 2(B) II show, respectively, plan view andperspective view of another embodiment of the present invention;

FIG. 2(C) II, FIG. 2(D), and FIG. 2(E) are, respectively, plan views ofthe other embodiments of the present invention;

FIG. 2(C) I is a side view of the embodiment of FIG. 2(C) II;

FIG. 3(A)-FIG. 3(E) are electrical connection diagrams showing,respectively, equivalent circuits of thickness shear vibration, crystalelectromechanical filters wherein the principle of the invention isutilized;

FIG. 4(A) shows a schematic electrical connection diagram indicating thecomposition of a polarized bandpass filter through the combination ofelectrical elements and the electromechanical filter of the invention;

FIG. 4(B) shows an equivalent connection diagram of the embodiment ofFIG. 4(A) FIG. 5 is a graphical representation indicating examples ofmeasured values relating to electromechanical filters according to theinvention having various pass band widths; and

FIG. 6 is a graphical representation indicating one ex ample of therelationship between the size of a crystal plate of the invention andthe pass band width.

Now referring to the accompanying drawing, the principle of operationwill be described in detail. In FIG. 1 which shows views of vibrationaldisplacement in thickness shear mode with a rectangular plate taken asan example, the thin crystal plate has its sides on X-axis and Z- axis.The X-axis in the figure corresponds to the crystallographic X-axis(electrical axis) of the crystal, and the Z -axis in the figure issomewhat deviated from the crystallographic Z-axis (optical axis) of thecrystal. This angle of deviation, Z-Z represents cutting orientation,and although varying slightly with dimensions and frequency, it is 3515and 49, respectively, for AT cut and BT cut, well known as the cutangles of zero temperature coefiicient. Experimentally, it happenssometimes that a vibration of shorter wave is superposed upon theelectrical polarization distribution of thickness shear mode shown inFIG. 1, and this is due to the mechanical coupling of theafore-mentioned higher flexural vibration with the so-called thicknessshear vibration. However, as is apparent by macroscopic observation, theX-axis direction has several sine waves and the Z -axis directionperpendicular to the plane of the X-axis has several cosine waves (onecosine wave shown in FIG. l(C)) or an approximately uniform electricalpolarization distribution. Vibrations of FIG. 1(A) and (B) correspond tozero cosine wave distribution. In other words, all these vibrations areessentially the same thickness shear vibrations, but numerous frequencyvibrations may appear with different combinations of several sine wavesdistributed in the X-axis direction and several cosine waves distributedin the Z -axis direction. FIG. 1 shows three-dimensional representationsof the distributions of these components, that in the case of FIG. 1(A)being semi-circular.

This electrical polarization distribution is similar tothe vibrationaldisplacement distribution. The thickness shear vibration is a pure shearvibration, with substantially all vibrational displacements in theX-axis direction. The displacement has sine wave form in the X-axisdirection and cosine wave form in the Z -axis direction.

The Y -axis in the figure (mechanical axis) lies in a direction which isdetermined by the frequency required, that is, the thickness directionof the crystal plate. So far as the fundamental frequency is concerned,the thickness dimension is determined uniquely. The vibrationaldisplacement in the Y -axis direction shows one cosine wavedistribution.

From the foregoing description, it will be seen that the vibration ofFIG. 1(A) has a vibration displacement dis tribution with one sine wavein the X-axis direction, one cosine wave in the Y axis direction, andzero cosine wave in the Z -axis direction. This fundamentalthickness-shear mode will be denoted by the symbol (1.1.0). Following asimilar notation, the inharmonic thickness-shear mode of FIG. 1(B) and(C) can be expressed as (2.1.0) and (1.1.1), respectively. It will beobvious that numerous vibrations of other modes are conceivable, but inthe general crystal oscillators which are excited in thicknessdirection, it is possible for some to occur and not possible for othersto occur, depending upon the condition in which electrodes are attached.Generally speaking, a crystal vibrator has electrodes attachedover theentire surface or partially on the central area, and, therefore, itsexcitation for a vibrational distribution with even number of sine wavesin all X-axis, Y -axis and Z -axis directions is impossible ordifficult. Vibrations of odd order appear then as unwanted response, ingeneral.

Resonance frequencies of the above-mentioned thickness shear vibrationswill now be examined. As is well known, equations of motion are asfollows:

6X OX am Where A, B and C are the constants of integration, independentof. position and time. Since the boundary condition in that externalforce is zero at x=x y=y and where p, q and r are positive integers, andx y and 2 are the dimensions of the crystal plate in the X-axis, Y,-axis and Z -axis directions, respectively. The numbers, p, q, and r,correspond to the numbers of the afore-mentioned sine and cosine waves,and 2:1, q=1 and r=0 for the mode (1.1.0). For the condition y x Z andalso for the close proximity of the main thickness shear vibration, theresonance frequencies can be expressed from the foregoing equationsapproximately as Cn 055 0'66 0 l/n 66, 0. (A)

where C C and C are the adiabatic elastic constants of the crystal whenit is deflected. Thus, numerous frequencies of thickness shear vibrationmodes are determined. Of these frequencies of unlimited number, twoparticular frequencies are utilized for use in the band pass filter asan embodiment of this invention.

FIG. 2(A) to FIG. 2(E) show examples of construction of a few crystalmechanical filters according to the invention. The rectangular plateform of FIG. 2(A), which has the simplest construction, consists ofelectrodes plated on both surfaces of a crystal plate 1, electrodes 2and 3 being formed on one surface in positions opposite electrodes 4 and5 formed on the opposite surface in the case illustrated. The electrodesmay be plated onto the crystal plate 1 by any of such methods as avacuum evaporation method or a chemical plating method.

The electrode plates are formed over the entire surface of the crystalplate or in any desired form, for instance, a circle or a rectangle, anddivided into two parts with proper attention to such considerations asthe equivalent inductance to be described hereinafter and suppression ofunwanted vibration. At the corners of the electrodes, by attachingsilver points by a baking method or by the clip mount method, lead wires6, 7, 8, and 9 are formed. These lead wires are adapted to maintainelectrode conductivity and, at the same time, to serve as support wires.These lead wires 6, 7, 8 and 9 are made to be the terminals, 11, 12, 13and 14 shown in FIG. 3(A), and the crystal plate 10 is made to be theafore-mentioned crystal vibrator. The direction of vibration, that is,the X-axis direction, is made to coincide with the transmissiondirection, the input terminals being 11 and 12, and the output terminalsbeing 13 and 14. The electrodes are divided along a center line in thedirection perpendicular to the direction of vibration.

The equivalent electrical circuit of this crystal vibrator will now beconsidered, beginning first with the fundamental thickness-shear mode(1.1.0), which, as described above, has one sine wave in the X-axisdirection and uniform distribution in the Z -axis direction. Terminals11, 12, 13, and 14 in FIG. 3(A) are assumed to be represented by 1, 2,3, and 4 in FIG. 3(B). Then, the equiva' lent circuit is found tocomprise: electrical capacitance C (capacitance of crystal plateregarded as a dielectric material); C C C C (capacitamces betweenterminals 1 and 3, 1 and 4, 2 and 3, and 2 and 4); an electromechanicaltransducer 1z (Where is the electromechanical conversion factor), andmechanically equivalent mass L and stiffness C If the mechanicalconstants, L and C are transformed into electrical constants, L and Cthe transducer 1: is transformed into an ideal transformer 1:1, andinput terminals 1a and 2a and output terminals 3a and 4a are newlyestablished, this circuit may be transformed into an equivalent, purelyelectrical circuit of lattice type as shown in FIG. 3(C). Here, L and Care the most important elements in the design of the filter according tothis invention and are determined by the dimensions of the vibrator andalso by the spatial distribution of electrodes on the vibrator surface,That is, with a vibration mode of (1.1.0), and a crystal plate ofdimensions x y and z The equivalent inductance L may be expressed as(til from which it can be seen this equivalent inductance varies withthe electrode distribution and crystal size.

Next, the inharmonic thickness-shear mode (2.1.0) shown in FIG. 1-(B),which has two sine waves in the X-axis direction and uniformdistribution in the Z -axis direction will be assumed for the instantvibrator. For the general vibrator, excitation in this mode isimpossible or difficult, whereas, for the resonator according to thisinvention, in which each electrode plate is divided into two parts, andthe electric charges on the respective vibrating surfaces are separatelyled out, it is found, by the same consideration as for FIG. 1(A), thatthe (2.1.0) mode vibration is possible with the phase of output ter-=minals 3 and 4 in anti-phase with respect to the previous case. Thisfact is an important condition as will be described hereinafter. Theequivalent electrical circuit is a lattice type circuit which comprisesequivalent inductance L a and equivalent capacitance C 11, both derivedfrom the resonator, capacitances C C C C and C all exactly the same asthose for the previous case, and an ideal transformer 1:1, with inputterminals 1b and 2b and output terminals 3b and 4b as shown in FIG.3(D).

The equivalent inductance -L a and the equivalent capacitance C a arecalculated in the same manner as described for the previous case. Ifeach electrode plate is attached over the entire crystal surface anddivided into two parts, the parts carrying the same electric charges,that is, the terminals 11 and 14 and terminals 12 and 13 of FIG. 3(A)are connected together, respectively, to form a two-terminal network.The equivalent constants La and C a thus calculated have approximatelythe same values as calculated by Eq. 5. This fact also is an importantcondition as will be described hereinafter and makes a symmetricallattice type band pass filter feasible.

The two modes of vibrations so far described are, in effect, present inone and the same vibrator, and, there force, the input terminals 1a and2a and the output terminals 3a and 44; for one vibration are essentiallythe same as 1b and 2b and 3b and 4b, for the other. Thus, by consideringonly the two modes of vibration, the two equivalent electrical circuitsderived above can be com-= bined to form the circuit shown in FIG. 3(E).

FIG. 3(E) represents a lattice circuit type band pass filter. If theimpedance of -a series arm comprising 2L a, C a/2, and C /2+C and theimpedance of a lattice arm comprising 2L C 2 and C /2+C are denoted by Zand Z respectively, the image transfer constant, the image impedance andthe characteristic function are given. 'by respectively the followingwell-known equations A point of zero attenuation, which satisfies Z -Z=1, is the ideal pass point, and inside the pass band, at least 2,, and2,; should have dissimilar signs. Also, a point of infinite attenuationsatisfies Z =Z and inside the attenuation region, 2,, and Z should be ofthe same sign. By the application of the reactance theorem, Z and Z arethen expressed as:

where H, and H are constants, in is the angular frequency, m is theupper cut-off angular frequency and w is the lower cut-off angularfrequency, that is, w a w;. The pass band width A is given by:

With reference to FIG. 3 (E), the required band pass filter will beobtained if Zr, and Z correspond to Eqs..10 and 11, w to the resonancefrequency of L 0 w to the anti-resonance frequency of Z and resonancefrequency of L aC a, and w to the anti-resonance frequency of Z For thefundamental thickness-shear mode (1.1.0), the resonance frequency of L Cis obtained by placing 7:1, q=1 and r=0 in Eq. 4, as mentionedhereinbefore. For the resonance frequency of L aC a, the vibration mode(2.1.0) is utilized, and this frequency is obtained by placing p=2, q=1and r=0 in Eq. 4. The results are:

Then, w,,w determines /2 of the band width of the symmetrical latticetype band pass filter. Since, however, the two resonance frequencies, asevident from Eqs. 7 through 11, have no other resonance frequency inbetween, the resonator uses only the vibrations of adjacent resonancefrequencies. In other words, the resonance frequency of the fundamentalthickness-shear mode (1.1.0) and the resonance frequency of theinharmonic thicknessshear mode (2.1.0) could be two adjacent resonancefrequencies determined by dimensions or special method of fabrication,and their difference becomes /2 of the socalled band width. If thisdifference is denoted by B, it is given as:

B=f2mf1w TQ;'(Z Z fun (14) where f and f are the resonance frequenciesof vibration modes (2.1.0) and (1.1.0), respectively. The dimension y isdetermined by the required center frequency, while x can be designedaccording to the required band width, as seen in the equation above.

While in the above disclosure, vibrations of (1.1.0) and (2.1.0) modeswere described, analysis in exactly the same manner can be made relativeto the vibration of (1.1.1) mode shown in FIG. 1(C). For instance, ifthe vibration of 1.1.0) mode and the vibration of (1.1.1) mode are used,it is obvious from the foregoing description that the (1.1.1) vibrationhas an anti-phase relationship with respect to the (1.1.0) vibration.

In this case, it will be obvious that the dividing direction of theelectrode is different from that in the previous case, and the electrodeplate should be divided in the Z,- axis direction into two parts inaccordance With the distribution of electric charge.

As mentioned above, a band pass filter can be constructed by using asingle vibrator, and the filter thus constructed is of a constant K typein which the attenuation pole, that is, the infinite attenuation point,is extremely remote, as given by Eq. 9 owing to L L a and C =C However,by the procedure as described below, it is pos sible to obtain apolarized type band pass filter in which the attenuation pole is broughtnearer to the cut-off region. FIG. 4(A) shows the same resonator asdescribed before, except that two electrodes on one side are connectedto a common terminal 24, while the other two are connected to inputterminal 1 and output terminal 3, separately. Combining two electrodeson one side into one results in a three-terminal resonator, which can befabricated relatively easily. In this case, a capacitance C is insertedbetween input terminal 1 and output terminal 3. The same relation isobtained also when, in the four-terminal circuit of FIG. 3(C) acapacitance is inserted between the terminals 1a and 3a. The equivalentelectrical circuit is shown in FIG. 4(B), where C is inserted inparallel with Z in the series arm.

The attenuation characteristic can be calculated in the followingmanner. First the following will be considered:

Then, for the region where and for w in the vicinity of w 1 2P SZ 1 (16)whereby an attenuation pole is produced at a point of 9m, and theattenuation at this time, from Eq. '9, becomes Eq. 16 is obtained byplacing the denominator or of Eq. 17 equal to zero, and at the S2,.point, the attenuation becomes infinity.

The fact that a polarized type electromechanical filter can beconstructed in a simple manner by introducing such electrical elementsas a capacitance, or a stray capacitance between lead wires 1 and 3,etc. is also one of the unique features of this invention.

Calculations based on approximate solutions of vibrational displacement,etc., have been shown for the case of rectangular plate vibrator, butthe concept is equally applicable to the circular or other plates ofmore complicated construction presented in the illustrative embodiments.

The principle of the thickness shear electromechanical filter accordingto the invention now having been de scribed, a. few illustrativeembodiments are presented below.

FIG. 2(A) shows an embodiment wherein a rectangular plate is used. Thecrystal plate 1a having dimensions designed according to the principledescribed is provided with electrodes 2, 3, 4, and 5. In effect, acommon electrode, not divided, is used for 4 and 5. The electrodes 2 and3 which are formed by division, are connected to in put terminal 6 andoutput terminal 7, respectively.

A feature (if this simplified construction is easy fabrication. Thecrystal plate shown in FIG. 2(B) is the same as that of FIG. 2(A) exceptthat the dimension perpendic ular to the transmission direction is verysmall. An important feature of this case, where the transmissiondirection is taken on the X-axis is that the dimension in the Z axisdirection is selected small enough to minimize the (1.1.1) vibration forutilization of vibration modes (2.1.0) and (1.1.0). This reduction in Zdimension is necessary because a square plate entails the presence ofthe resonance frequency of (1.1.1) mode between the (1.1.0) and (2.1.0)vibrations. However, this vibration of (1.1.1) mode cannot occurelectrically provided that the electrode is attached exactly in thecenter, but if any error exists in this electrode position, somevibration may occur.

The crystal plate of FIG. 2(C) has a circular form. This is advantageousowing to its very large effect of. suppressing other vibrations, easyfabrication, and re producibility. The crystal plates of FIG. 2(D) andFIG. 2(E) are the same as that of FIG. 2(C), except that narrow cuts 20are made on the circular plate as a means for suppressing othervibrations. As described hereinbefore, the thickness shear vibration isaccompanied by numerous vibration modes represented by pqr, and thehigh- I er fiexural vibration also has numerous resonance frequencies.For the suppression of these unwanted vibrations, narrow cuts made asshown in the figure are highly effective. Further, it is necessary toprovide optimum electrode area by making the electrode smaller than thecircular plate so that electrical excitation of unwanted 8 vibration maybe prevented. The crystal plate of FIG. 2(E) is provided with cuts ofwedge form in place of the narrow cuts. The purpose is, here again, thesuppression of unwanted vibration.

As to the effect of narrow cuts, wedge cuts, or cuts of other form,exact calculation by theory is dlfilCUlt, but, experiments indicate thatthese cuts are effective for suppression of unwanted vibration.

The results of measurement on the thickness shear vibrationelectro-mechanical filters constructed on the principles described andin the constructional forms shown, are presented in the followingdescription. FIG. is a graph of measured attenuation of a band bassfilter of 10.7 mc. center frequency constructed by using a circular ATcut vibrator provided with suppression of unwanted vibration. Theterminal impedance used is a resistance calculated by Eq. 8 for thecenter frequency. The thickness dimension of the crystal plate isapproximately 0.156 mm. for the center frequency of 10.7 me. The bandwidth as a polarized type is kc., kc., or kc., as determined by thediametric dimension of the crystal plate, and the relation is shown inFIG. 6.

In quite a manner similar to that of the general filters, severalsections of such electromechanical filters are cascade connected toobtain the required amount of attenuation. The limit of maximumavailable band width of this filter is determined by the capacitanceratio P of Eq. 15. However, for wider band width, the design can be madewith a parallel circuit of coil and capacitor connected between sectionsso as to compensate for C Since the basic material of a thickness-shearvibration electromechanical filter according to the present invention asdescribed above is a crystal body, the said electromechanical filter ishighly advantageous on such points as temperature characteristics,changes with time, frequency stability, frequency adjustment,high-frequency characteristics, losses, abrupt cutoff characteristics, Qvalue, and possibility of micro-miniaturization.

While the foregoing description has been presented entirely on the basisof a crystal body as the basic material, it will be obvious that theabove-described operation, principle, and construction will beapplicable with similar effect in the case wherein another substance ofcomparable characteristics is used.

Although this invention has been described to a few particularembodiments thereof, it is not to be so limited as changes andmodifications may be made therein which are within the full intendedscope of the invention, as de fined by the appended claims.

What is claimed is:

1. A high frequency electromechanical band pass filter comprising a thinpiezoelectric crystal having an X axis (electrical axis), Y axis(mechanical axis) and Z axis (Optical axis) and having thecharacteristic of vibrating in the thickness shear mode of vibrationwith substantially all vibrational displacements in the X axisdirection, said crystal having opposite parallel major faces, and aplurality of electrodes on said major faces, said electrodes comprisingan input electrode and an output electrode on one of said major facesdivided from one another along a center line perpendicular to thedirection of the X axis or Y axis of said crystal with parallel edges ofsaid electrodes spaced from one another, the outer dimensions of saidcrystal being selected to determine the desired center frequency andband width of said filter, and notches being provided in opposite edgesof the perimeter of said crystal in line with the division between saidinput and output electrodes for suppression of unwanted vibrations.

2. A high frequency electromechanical band pass filter comprising a thincircular piezoelectric crystal having an X axis (electrical axis), Yaxis (mechanical axis) and Z axis (optical axis) and having thecharacteristic of vibrating in the thickness shear mode of vibrationwith substantially all vibrational displacements in the X axisdirection. said crystal having opposite parallel circular major facesand electrodes on said opposite major faces including a plurality ofapproximately semi-circular electrodes on one of said major faces, saidelectrodes being of materially smaller radius than said crystal and saidsemi-circular electrodes comprising an input electrode and an outputelectrode disposed on one of said major faces and having adjacentdiarrletral edges spaced apart and approximately perpendicular to thedirection of the X'axis or Y axis of said crystal and to the directionof transmission from said input electrode to said output electrode, saidpiezoelectric crystal having an annular portion of appreciable radialdimension between said electrodes and the periphery of said crystal tosuppress unwanted vibrations.

3. A filter accordingto claim 1, in which notches are provided in theperimeter of said crystal in line with the space between the diametraledges of said electrodes.

4. A filter according to claim 3, in which said notches extend inwardlysubstantially to the perimeter of said electrodes.

5. A high frequency electromechanical band pass filter comprising a thincircular piezoelectric crystal having the characteristic of vibrating inthe thickness shear mode of vibration, said crystal having oppositeparallel major faces, and a plurality of electrodes on said major facescomprising two approximately semicircular electrodes disposed on thesame major face and having diametral edges spaced apart and parallel toa diameter of said crystal,

10 notches being provided in the perimeter of said crystal in line withthe space between said electrodes for suppression of unwantedyibrations.

6. A filter according to claim 5, in which said electrodes are ofsmaller radius; than said crystal and said notches extend inwardlysubstantially to the perimeter of said electrodes.

References Cited UNITED STATES PATETNS 2,037,171 4/1936 Lane 333722,097,458 11/ 1937 Hansell 33372 2,199,921 5/1940 Mason 33372 2,300,07510/ 1942 Sykes 33372 2,284,753 6/1942 Mason 310-9.6 2,301,828 11/1942Stone 33372 2,306,909 12/1942 Sykes 33372 2,309,467 1/ 1943 Mason 333722,373,431 4/1945 Sykes 33372 2,429,639 10/ 1947 McSkimmin 333722,799,789 7/ 1957 Wolfskill 3109.4' 3,185,943 5/ 1965 Honda et al. 333723,222,622 12/1965 Curran et al. 33372 3,297,968 1/1967 Fowler 33372HERMAN KARL SAALBACH, Primary Examiner.

c. BARAFF, Assistant Examiner.

